Rare Collection of Whale Fetuses Reveals the Evolution of Cetacean Hearing

For evolutionary biologists, whale ears are a strange and fascinating enigma. Studying them as they develop in the womb can help us understand how they evolved. But collecting whale fetuses today is out of the question because hunting whales is illegal, and scientists have to rely on strandings, which don't often turn up pregnant females.

So when Smithsonian researcher Maya Yamato first learned of a collection of 56 whale fetuses preserved in alcohol at the National Museum of Natural History, she trundled them off to the museum’s CT scanning lab. “We are never going to get this kind of material again,” says Yamato, “Since they’re so valuable and rare, it’s not desirable to dissect them.”

Yamato, a postdoctoral fellow working with curator of fossil marine mammals, Nicholas Pyenson, wanted to get a better look at how ears developed in whale fetuses. The collection includes some 100-year-old specimens, with most dating back to a period in the early and mid-20th century, when commercial whaling operations were thriving. In 1986, the hunting of whales was banned by the International Whaling Commission, and although some of the specimens in the collection come from by-catches and strandings, many are associated with the whale hunting of an earlier era.

“They’re unique because there are species that no one will ever be able to collect again,” says Pyenson. “In some cases either the circumstances under which they were collected can’t be replicated, or the organisms aren’t there in the wild.”

Because dissecting any of the specimens or performing any sort of invasive examination was not an option, Yamato used the non-invasive CT technology to inspect the very delicate features inside the whales’ skulls. The results of Yamato’s work appear in a new study today in the journal PLOS ONE. What Yamato found helps to confirm what the fossil record says about how whales made the dramatic shift from land to sea, and how whale families continued to evolve divergent physical characteristics to accommodate different lifestyles in their underwater environment.

Cetaceans have evolved into two distinct groups: those with teeth and those with baleen—a bristly sieve-like structure made out of keratin. Toothed whales such as sperm whales, orcas and dolphins hunt and catch prey in their toothy jaws, but baleen whales such as humpbacks, blues and bowhead whales gulp water and filter it through their baleen to trap small fish and krill.

Not only do they feed in completely different ways, but the two groups of whales hear differently, too. The groups process sound on the extreme opposite ends of the frequency scale for mammalian hearing. Baleen whales use ultra-low frequency sounds to communicate over long distances. They hear and vocalize using frequencies lower than those used by any land mammal, even elephants.

Toothed whales, on the other hand, hear and produce sounds in frequencies higher than the little brown bat, the highest pitched land mammal. Toothed whales rely on that ability for echolocation to navigate and to find prey.

Using the CT scans of the fetal whale collection, Yamato and Pyenson were able to trace the development of these two very different types of whales. “If the Smithsonian didn’t have a CT scanner, and it didn’t have the largest marine mammal collection in the world, this study probably would not have happened,” Yamato said.

She and Pyenson are the first scientists to identify the developing “acoustic funnel” in the ears of a fetal specimen. Whale ears are entirely internal. They rely on fatty tissue associated with the cone-shaped acoustic funnel, which is found only in whales. Exactly how they work is not yet fully understood. In all toothed whales, the acoustic funnel is forward facing, but in some baleen whales, the funnel is oriented toward the side of the head.

The early stages of ear development in the fetuses parallels the evolutionary divergence of cetaceans from their land-based ancestors. And, even more importantly, the divergence of the two groups from one another. The acoustic funnel develops similarly, at first, in all cetaceans. Then later, as the fetuses of the two groups develop, it changes orientation and shape in the brain, mimicking the evolutionary process.

“We want to be able to trace out all the structures within the ear.” Yamato says. “In early fetuses, we see the typical mammalian structures. They are more similar to all other mammals—to land mammals. With this non-invasive method of studying these rare specimens, we are able to observe how those typical land mammal ingredients are rearranged to form the ears of modern whales.”

This kind of information is tough to get. Ancient fetuses don’t preserve well in the fossil record. Pyenson says there is just one known fossilized specimen of an unborn whale. So examining these historical specimens that included 15 different species of both toothed and baleen whales was like opening a treasure trove of data. Now that the scans are available to the scientific community, other scientists are already starting to use them to study other features of whale development.

“How we grow, especially in utero, tells us a lot about how we have evolved,” says Pyenson. “So looking at fetal data can give us a lot of answers in terms of how an animal develops.”

“This kind of study helps to illustrate the incredible diversity and adaptation of life on Earth,” Yamato adds. “Although we all descended from the same ancestor, we have different strategies for dealing with very different environments.”

Kimbra Cutlip | | READ MORE

Kimbra Cutlip is a freelance science writer, covering natural history, atmospheric sciences, biology and medicine. She is a contributing editor for Weatherwise magazine.